An independent variable is a variable whose value can either occur freely, be chosen freely, or be manipulated freely. In other words, an independent variable can usually be anything you want it to be.
For example, suppose you go to the mall to buy grapes. The retail price says "2 dollars per pound."
The number of pounds of grapes that you buy is an independent variable.
It is so because you are free to buy 1 kilogram, 3 kilograms, 100 kilograms, or any other amount that you like as long as the store has this amount that you need in stock.
1. A recipe says to use 1/4 teaspoon of pepper for each pound of turkey meat.
The number of pounds of turkey that you use is an independent variable.
2. A car can travel about 30 miles on one gallon of gas.
The number of gallons of gas is an independent variable.
3. Let C = 1.02X be the cost of producing an item with X being the number of items produced and C the cost.
X is an independent variable.
You are free to choose or manipulate freely the number of items you produce.